The flt3 ligand: a hematopoietic stem cell factor whose activities are distinct from steel factor.

A number of growth factors have been described that affect the hematopoietic system. Among this group are Steel factor (also known as mast cell growth factor, stem cell factor and kit ligand), and the more recently described flt3 ligand. These factors have been shown to function by binding to and activating the c-kit and flt3 tyrosine kinase receptors, respectively. Both of these factors stimulate the growth of mouse and human hematopoietic progenitor cells. These factors therefore differ from such later acting hematopoietic factors as colony-stimulating factor (CSF)-1, which regulates the growth, survival and differentiation of monocytic cells through the c-fms tyrosine kinase receptor. Like Steel factor, the flt3 ligand has little biological activity on its own, but synergizes well with a number of other colony stimulating factors and interleukins. One major difference between the two factors appears to be their effect on mast cells. Steel factor stimulates both the proliferation and activation of mast cells, while preliminary data with the flt3 ligand suggests that it has no effect on mast cells. Although the flt3 ligand and Steel factor each act on early hematopoietic cells, differences in their activities suggest that they are not redundant and are both required for normal hematopoiesis.